1. Background Art
Electromagnetic interference has increased in recent years due to advances in the performance of electrical and electronic equipment and also to the growing use of the same. Even such displays as CRTs and PDPs generate electromagnetic waves. A PDP is an assembly composed of a glass substrate having a data electrode and a fluorescent layer, and a glass substrate having a transparent electrode. When operated, such a display not only emits visible light from which an image on the display is produced, but also generates electromagnetic waves, near infrared rays, and heat in large amounts. In general, a front panel containing an electromagnetic wave shielding sheet is mounted on the front of a PDP in order to shield the electromagnetic waves that the PDP generates. Such a front panel is required to have an efficiency of 30 dB or more in shielding electromagnetic waves with frequencies of 30 MHz to 1GHz that are emitted from the front of the display. The front panel is also required to shield near infrared rays with wavelengths of 800 to 1,100 nm emitted from the front of the display because these rays cause malfunction of other equipment such as VTRs.
Further, in order to keep an image displayed on the display highly visible, the electromagnetic wave shielding material is required to be less visible (non-recognizability being high) and to have, as a whole, moderate transparency (visible light transmission).
Furthermore, PDPs are characterized by having large-sized screens, and electromagnetic wave shielding sheets for such PDPs are large in size (external dimension); their sizes are as large as 621×831 mm for 37-inch displays and 983×583 mm for 42-inch displays, for example, and still larger sizes exist. This fact demands a production method that is convenient to handle large-sized materials. In sum, the electromagnetic wave shielding sheets are required to have electromagnetic wave shielding properties and to be made from less visible electromagnetic wave shielding materials having moderate transparency so as not to impair the visibility of images displayed, and there is a demand for a production method by which an electromagnetic wave shielding sheet can be produced in a small number of steps with high productivity.
2. Related Art
To produce electromagnetic wave shielding sheets containing metal mesh layers, the following two methods have usually been employed.
One of the known methods is that electrically conductive ink or a photosensitive coating liquid containing a catalyst for chemical plating is applied to the entire surface of a transparent substrate, and the coating is photolithographically made into a mesh, which is then plated with a metal (see Patent Documents 1 and 2, for example). However, this method is disadvantageous in that, since the metal layer face on the transparent substrate side cannot be blackened, it is impossible to avoid lowering of image visibility that is caused by extraneous light such as sunlight that is incident on and is reflected from the metal mesh. Further, in the production process, if electrically conductive ink is used, it takes a longer time for plating because the electrical resistance of the electrically conductive ink is high, and productivity is therefore low. Besides, this method has been disadvantageous also in that the metal mesh is oxidized in the air and undergoes change of properties to have an increased electrical resistance, which results in deterioration of electromagnetic wave shielding properties.
Another known method is as follows: a copper mesh layer with line parts that define multiple openings is laminated to a PET film (transparent substrate) with an adhesive layer; and all of the surfaces, back surfaces, and side faces of the line parts of the copper mesh layer are subjected to blackening treatment (see Patent Document 3, for example). However, the blackening treatment is chemical conversion treatment, and needle crystals are formed, so that the blackening layer thus formed readily falls off or is deformed. Moreover, since the treatment is conducted at a high temperature, the laminate tends to be curled and its appearance is adversely affected.
Patent Document 1: Japanese Patent Laid-Open Publication No. 13088/2000,
Patent Document 2: Japanese Patent Laid-Open Publication No. 59079/2000, and
Patent Document 3: Japanese Patent Laid-Open Publication No. 9484/2002.